1. Field of the Invention
The present invention relates to a process and apparatus for filling tubular plate positive electrodes for use in lead-acid secondary cells with lead oxide particulate material in a highly fluid slurry form. According to the process of the present invention, tubular plate positive electrode assemblies comprising a plurality of substantially lead spines aligned on a conductive bar, each spine enveloped by a porous tubular sheath, are footed prior to being filled with lead oxide particulate material in a highly fluid slurry form. The foot according to the present invention has a plurality of aligned projections and slurry access ports. When the foot is installed on a tubular plate electrode assembly, each projection engages a spine in its spine recess and is itself engaged within a tubular sheath. Lead oxide particulate material in a highly fluid slurry form is introduced into the annular space between each spine and its corresponding tubular sheath through the slurry access ports penetrating the foot.
2. Description of the Prior Art
Lead acid secondary cells comprising two substantially lead electrodes in contact with sulfuric acid electrolyte are well known to the art and are utilized in a variety of applications, including starting, lighting and ignition batteries for internal combustion engines and electrical systems, stationary batteries for powering communications systems, electrical utilities, computer systems, and for providing emergency power, motive power batteries to provide power for propulsion of lift trucks, mining equipment, and the like, and special purpose batteries such as aircraft, marine, military, and small, sealed batteries for consumer use. Each of the electrodes must be provided with the appropriate active material prior to cell operation, since it is the active material which provides reactants for the reversible electrochemical reactions taking place during charge and discharge of the cell. The electrodes provide conductive support structures for the active material which act as current collectors for the cells.
Conventional negative electrodes for use in lead-acid secondary cells comprise lead or lead alloy grids. The open spaces and surfaces of the grid are pasted with a paste-like mixture comprising one or more oxides of lead which is subsequently reduced, forming the electrochemically active material known as sponge lead. Positive electrodes may comprise the same type of lead or lead alloy grids, or they may comprise a tubular plate arrangement wherein a plurality of individual lead or lead alloy spines are aligned on a conductive bar, each spine is enveloped by a porous tubular sheath, and active material, in the form of lead dioxide, is provided in the annular space between the spines and porous tubes, and is confined by the tubular sheath. Typically, a mixture comprising lead oxide compounds is introduced into the annular space, and the positive electrode assembly is then treated to convert the lead oxide compounds to the electrochemically active material, lead dioxide.
Tubular plate positive electrodes are conventionally prepared by pouring fine lead oxide powder into the annular space between each spine and its corresponding tubular sheath through the unobstructed, open end of the electrode assembly while the electrode assembly is being vibrated. This conventional method of filling tubular plate electrodes with active material has several serious shortcomings, and particularly serious is the environmental hazard of human exposure to the lead oxide powder. Utilization of fine lead oxide powders according to this conventional filling process presents a serious health hazard to those working in the general area of the filling operations, and also results in substantial materials losses. The cost of protecting personnel from the health hazards of airborne lead oxide dust are prohibitive, and recovery of airborne lead oxide powder for subsequent utilization is very difficult. Furthermore, filling tubular plate electrodes with lead oxide dust in this manner often results in non-uniform filling and electrodes filled in this manner may demonstrate unsatisfactory performance.
After filling the annular space between each spine and its porous tubular sheath with lead oxide powder according to conventional methods, the electrode is typically "footed" to seal the open ends of the tubular sheaths. The conventional methods of filling tubular plate electrodes with fine lead oxide powder require very precise regulation and metering of the lead oxide powder to ensure that no void spaces remain between the lead oxide powder level and the foot, which may extend for a short distance into the tubular sheaths, and to ensure that the electrode has not been overfilled with lead oxide powder so that the foot cannot be fully inserted. Precise regulation of the lead oxide powder filling level is difficult to achieve. In addition, the spines may become misaligned during filling with lead oxide powder, and subsequent insertion of the foot may bend the misaligned spines. All of the aforementioned shortcomings of conventional tubular plate electrode lead oxide powder filling methods adversely affect the performance and reliability of the electrode.
In view of the environmental hazards of conventional lead oxide powder filling methods and unsatisfactory performance of electrodes filled with lead oxide dust in the conventional manner, several alternative techniques have been proposed for providing tubular plate electrodes with active material. U.S. Pat. No. 4,076,058 teaches a method and apparatus for introduction of active material in an aqueous slurry form into the porous envelopes of tubular plate electrodes. This patent teaches introduction of a liquid slurry of low viscosity directly into the open end of porous envelopes of tubular plate electrodes at low pressure, with the arrangement of outlet nozzles of an injection manifold corresponding to the arrangement of tubular envelopes and each outlet nozzle of the injection manifold having an outer diameter corresponding to the inner diameter of the porous tubular envelopes. Each tubular plate must be clamped at a plate filling station and aligned with an injection manifold comprising rigid feed tubes prior to filling.
U.S. Pat. No. 3,885,988 teaches a process and apparatus for filling tubular plate electrodes with lead oxide material in a slurry form wherein supplementary water is added to a paste-like lead oxide material to form a suspension and the suspension is forced into the individual tubes under pressure to prevent settling of the particulate lead oxide material. A plurality of nozzles corresponding to the number and arrangement of tubes in the tubular plate electrodes are fitted into or over the open ends of the tubes during injection.
U.S. Pat. No. 3,949,793 teaches a method for filling tubular plate electrodes with an active material paste wherein the active material paste is pumped through an extruder head into the open tubes of a tubular plate electrode which are aligned and retained in a nozzle block, the nozzles fitting into the open ends of the tubular sheaths. U.S. Pat. No. 4,039,730 teaches a similar method for filling tubular plate electrodes with active material in a paste form wherein the paste material additionally comprises electrolyte-resistant fibers such as glass or plastic. The electrolyte-resistant fibers occlude pores in the tubular wall to retain active material within the tubular envelope, thereby reducing extrusion of the active material through the tubular envelope, and increasing the strength of the active material.
U.S. Pat. No. 4,146,063 teaches a method for filling tubular plate electrodes with an active material in paste form wherein the porous tubular sheath is filled with paste by centrifugation, surplus liquid being removed through the porous sheath during centrifugation.
U.S. Pat. No. 4,366,214 teaches a device for sealing the tubes of a tubular plate electrode comprising separate sleeves for each tube, each sleeve having a central through hole for retaining a spine. The sealing device is applied after the tubes have been filled with active material.
U.S. Pat. No. 4,011,370 teaches fixation of bottom strips to tubular plate electrodes prior to filling the tubular plate electrodes with active material in a powder form. To permit filling of the tubular sheaths after the bottom strip has been attached, the bottom strips are provided with hinged closure sections formed integrally with the base, which open to permit filling of the tubular sheaths with lead oxide powder and close tightly after the sheaths have been filled. The '370 patent teaches filling of the tubular plate electrodes with active material in a powder form only, and the hinged closure sections are especially suitable for use with lead oxide powder filling processes. This method does not ameliorate the environmental hazards inherent in lead oxide powder filling processes, and it does not prevent misalignment of the spines during the filling process. The '370 patent teaches the additional feature of integral projections extending from each end of the bottom strip to facilitate suspension or other automated handling of the electrodes subsequent to filling.
The disadvantages of prior art powder filling techniques, and particularly the environmental hazard, have been described above and are fully set forth in the U.S. patents relating to filling tubular plate electrodes with lead oxide materials in a paste or slurry form. Filling tubular plate positive electrodes with active material in a slurry or paste form is preferable from an environmental standpoint, but prior art processes are very costly due to the specialized equipment required to generate sufficient pressures for extruding paste or injecting slurry directly into each of a plurality of aligned tubular sheaths. Precise regulation is required to fill to the proper level so that the foot may be fully inserted without leaving any void spaces between the foot and the active material. Upright alignment of the spine during filling is difficult to maintain using prior art filling techniques. The prior art slurry and paste filling methods are generally unsatisfactory.